# Is there a special name for the orthogonal projection matrix onto the unit vector?

Many problems in multivariate analysis involve the $$n \times n$$ matrix:

$$\mathbf{M} \equiv \boldsymbol{I}_n - \frac{1}{n} \mathbf{1}_{n \times n}.$$

This is an orthogonal projection matrix onto the unit vector, so when it is applied to a column vector containing values $$Y_1,...,Y_n$$, this matrix subtracts the sample mean $$\bar{Y}_n$$ from these values:

$$\mathbf{Y} = \begin{bmatrix} Y_1 \\ \vdots \\ Y_n \end{bmatrix} \quad \quad \quad \implies \quad \quad \quad \mathbf{M} \mathbf{Y} = \begin{bmatrix} Y_1 - \bar{Y}_n \\ \vdots \\ Y_n - \bar{Y}_n \end{bmatrix}.$$

There are many simple properties of this matrix, owing to the fact that it is an orthogonal projection matrix. It has $$\text{tr}(\mathbf{M}) = \text{rank}(\mathbf{M}) = n-1$$, which means that it has a single zero eigenvalue and the remaining eigenvalues are all ones. It comes up a lot in multivariate analysis, including statistical problems, where it is common to look at random vectors after subtracting their sample means.

My question: Does this matrix have any special name? Is there a literature on this type of matrix? Aside from the properties I have listed here, are there any other important properties of this matrix that are useful in statistical problems dealing with random vectors?

• Maybe look at MDS or PCA literature. I vaguely recall it being called the mean-centering matrix there. – obscurans Jan 31 '19 at 2:59